1. Field of the Invention
The present invention relates to a battery pack structure with a heater, comprising a plurality of secondary batteries each of which has a power generating element and a battery case accommodating it, a housing case that houses the plurality of secondary batteries, and a heater.
2. Description of Related Art
Batteries such as nickel-metal hydride storage batteries have been watched as power sources of portable devices and power sources of electric vehicles, hybrid electric vehicles, and others.
However, the batteries such as nickel-metal hydride storage batteries have problems that discharge capacity is apt to decrease during cold conditions, failing to provide adequate output power. If such battery is used as a power source of an electric vehicle, a hybrid electric vehicle, or the like, for instance, it could not generate sufficient output power in low-temperature conditions, e.g., in a cold region where temperatures may fall to sub-zero.
In recent years, some techniques for solving the above problems by attaching a heater to a battery to heat the battery by use of a household power source have been proposed (e.g., Jpn. unexamined utility model publication No. 60 (1985)-192367 and Jpn. unexamined patent publication No. 2002-216731).
Jpn. unexamined utility model publication No. 60 (1985)-192367 discloses a battery pack structure with heater, in which a sheet heater is placed inside a bottom of a housing case made of a heat insulation material and two batteries are arranged in contact with the sheet heater in a container. Jpn. unexamined patent publication No. 2002-216731 discloses a heater system having a heater attached to a battery. The techniques disclosed in the above publications, in which the batteries are heated by use of the heaters, could improve the output characteristics of the batteries even in low-temperature conditions.
According to the techniques disclosed in the above publications including direct heating of the batteries by the heaters, the batteries tend to excessively rise in temperature if the temperatures of the heaters abnormally increase due to any malfunction or failure.
When a plurality of secondary batteries (a “secondary battery” in the following description represents a battery including a power generating element and a battery case accommodating the power generating element) is heated according to the method configured to directly heat batteries by use of a heater as disclosed in the above publications, there is a case where the battery temperatures widely vary among the heated batteries. This may be caused when some of the secondary batteries are out of contact with the heater, because the secondary batteries out of contact with the heater and the secondary batteries in contact with the heater are largely different in quantity of heat conducted from the heater. In such case, the output characteristics are largely different from one secondary battery to another. Thus, an entire battery pack could not produce stable output.